Electric power steering apparatus is a machine which operates the motor for generating the steering assist force according to a steering torque applied by a driver. Recently, such an electric power steering apparatus has been faced with a demand for high power operation. To meet this demand, there has been proposed an electric power steering apparatus wherein an auxiliary power supply independent from a battery is connected in series to the battery as needed so as to supply high electric power at high voltage (see, for example, Patent Document 1).
An electric double-layer capacitor, for example, may be used as the above auxiliary power supply. Such an auxiliary power supply must always be charged so as to be ready for use. However, it takes some time to charge the power supply having a low amount of remaining electric energy. Furthermore, charge control poses a substantial load on a control circuit for the electric power steering apparatus. It is therefore crucial to hold down the waste of the energy of the auxiliary power supply to a minimum. By the way, the auxiliary power supply is needed when the power supplied by the battery alone falls short. This means that the auxiliary power supply is used whenever the power from the battery falls short, regardless of the extent of such shortage of the power. If the auxiliary power supply operates at full capacity even when the power shortage is moderate, the auxiliary power supply is activated excessively as a result, and the energy of the auxiliary power supply is consumed more than necessary.